The DNAv4 protocol implementation in the DHCP component in Apple iOS before 6 sends Wi-Fi packets containing a MAC address of a host on a previously used network, which might allow remote attackers to obtain sensitive information about previous device locations by sniffing an unencrypted Wi-Fi network for these packets.
Apple Security Advisory 2012-09-24-1 - Apple TV 5.1 is now available and addresses issues relating to malicious media loading, memory corruption, and more.